Adhesive fillets and method and apparatus for making same

ABSTRACT

Methods of forming shaped adhesive fillets are disclosed. The adhesive fillets may be made by stacking multiple layers of adhesive sheets, consolidating the stacked layers, cutting strips from the consolidated layers, and forming the strips into adhesive fillets having desired cross sectional shapes. The fillets may be formed by feeding the strips through an apparatus comprising opposing rollers and a catcher, which facilitates release of the adhesive fillets from the rollers. The formed adhesive fillets are useful for applications such as stiffened panels for aircraft.

FIELD OF THE INVENTION

The present invention relates to adhesive fillets, and more particularlyrelates to methods and apparatus for making adhesive fillets forstructural applications such as aircraft stiffeners.

BACKGROUND INFORMATION

Adhesive fillets are used in applications such as aircraft stiffeners.For example, stiffeners with hat-shaped cross sections are oftenattached to the interior surface of aircraft skins in order to provideimproved mechanical properties. The adhesive fillets are positioned atinterior corners where the stiffeners meet the aircraft skin.

Conventionally, such adhesive fillets have been manually formed into rodshapes by hand-rolling an adhesive sheet, followed by consolidation in amandrel under heat and vacuum. Each adhesive rod takes several minutesto form, and the hand-rolled adhesive rods tend to have inconsistentshapes and quality.

The present invention has been developed in view of the foregoing and toaddress other deficiencies of the prior art.

SUMMARY OF THE INVENTION

The present invention provides shaped adhesive fillets for applicationssuch as stiffened panels for aircraft. The adhesive fillets are formedby stacking multiple layers of adhesive sheets, consolidating thestacked layers, cutting strips from the consolidated adhesive layers,and forming the cut strips into desired fillet shapes. The formedfillets may be used in stiffened aircraft panels, for example, at theinterior corners of hat-shaped stiffening members.

An aspect of the present invention is to provide a method of making anadhesive fillet. The method comprises stacking multiple adhesive layers,and forming the adhesive layers into a fillet shape.

Another aspect of the present invention is to provide a formed adhesivefillet comprising multiple stacked adhesive layers.

A further aspect of the present invention is to provide an apparatus formaking adhesive fillets comprising at least one roller structured andconfigured to form an adhesive strip into an adhesive fillet shape.

Another aspect of the present invention is to provide a stiffened panelincluding a stiffening member attached to the panel and at least oneformed adhesive fillet attached to the panel and the stiffening member,wherein the at least one formed adhesive fillet comprises multiplestacked adhesive layers.

These and other aspects of the present invention will be more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric sectional view of a stiffened panel includingadhesive fillets in accordance with an embodiment of the presentinvention.

FIG. 2 is an isometric view of an adhesive sheet conventionally used tomake adhesive fillets.

FIG. 3 is an isometric view of a rolled fillet rod made from an adhesivesheet as shown in FIG. 2.

FIG. 4 is an isometric view of a layup of multiple adhesive sheetsutilized in accordance with an embodiment of the present invention.

FIG. 5 is a partially schematic side view of an assembly forconsolidating the multi-layer adhesive layup shown in FIG. 4.

FIG. 6 is an isometric view of an adhesive bar cut from a consolidatedmulti-layer adhesive layup in accordance with an embodiment of thepresent invention.

FIG. 7 is a partially schematic side view of an adhesive fillet formingapparatus in accordance with an embodiment of the present invention.

FIG. 8 is a cross-sectional view of an adhesive fillet produced inaccordance with an embodiment of the present invention.

FIG. 9 is a sectional end view illustrating the assembly of a hat-shapedstiffening member on a skin panel, including the use of adhesive filletsof the present invention.

FIG. 10 is a side sectional view of an opposing roller adhesive filletforming apparatus in accordance with an embodiment of the presentinvention.

FIG. 11 is an end sectional view of the opposing roller adhesive filletforming apparatus of FIG. 10.

FIG. 12 is an isometric view of a fillet catcher in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a stiffened structure 10 including a panel 12 and astiffening member 14. The panel 12 may be made of any suitable materialsuch as metals or composites. For example, the panel may comprise agraphite/epoxy composite including unidirectional or woven graphitefibers in an epoxy resin matrix. In one embodiment, the panel 12 maycomprise an aircraft skin having a typical thickness of from about 0.05to about 0.2 inch or more. As a particular example, the aircraft skinmay have a thickness of from 0.10 to 0.13 inch. The stiffening member 14may be made of materials such as metals or composites, e.g.,unidirectional or woven graphite fibers in an epoxy resin matrix, andmay be dimensioned as desired for a particular application. For example,the stiffening member 14 may have a thickness of from about 0.05 toabout 0.2 inch or more, a width of from about 1 to about 6 inches, aheight of from about 0.05 to about 2 inches, and a length of from about1 to about 8 feet or more. As a particular example, when used as anaircraft skin stiffener, the member 14 may have a typical thickness offrom 0.11 to 0.126 inch, a width of from 2 to 4 inches, a height of from0.7 to 1 inch, and a length of from 18 to 36 inches. The stiffeningmember 14 shown in FIG. 1 has a hat-shaped cross section. However, otherstiffening member shapes may be used in accordance with the presentinvention, such as L-shapes, I-shapes, J-shapes, T-shapes, Z-shapes andthe like.

As shown in FIG. 1, the stiffened structure 10 includes adhesive fillets16 at the interior corners where the stiffening member 14 meets thesurface of the panel 12. In accordance with the present invention, theadhesive fillets 16 are made by a process of stacking, cutting andforming a selected number of multiple layers of adhesive material, asmore fully described below.

FIGS. 2 and 3 illustrate a conventional method for making adhesivefillets. The process starts with an adhesive sheet or strip 20 as shownin FIG. 2. The adhesive strip 20 typically has an approximate thicknessof about 0.014 inch, a width of about 1 inch, and a length of about 36inches. As shown in FIG. 3, the adhesive strip 20 is hand rolled into arod 22. In accordance with conventional processes, the hand-rolled rod22 may be placed in a V-shaped mandrel under vacuum and may be subjectedto heat and manual pressure in order to at least partially consolidatethe adhesive rod 22.

FIG. 4 illustrates a multi-layer adhesive structure 30 of the presentinvention including multiple adhesive layers 30 a-30 e. Five adhesivelayers 30 a-30 e are shown in the embodiment of FIG. 4. However, othernumbers of layers may be used depending on the particular application.For example, from 5 to 8 layers may be desirable. Each of the layers 30a-30 e of the multi-layer adhesive structure 30 may be made of amaterial such as epoxy adhesive sold under the designation FM300 byCytec, or the like. Materials such as scrim cloth, short chopped fibers,unidirectional graphite fibers and fillers may optionally be embedded ineach layer 30 a-30 e, or positioned between each layer. The thickness ofeach layer 30 a-30 e preferably ranges from about 0.01 to about 0.02inch. A particularly preferred thickness is about 0.014 inch The totalthickness of the multi-layer adhesive structure 30 typically ranges fromabout 0.05 to about 0.2 inch or more, more preferably from about 0.07 toabout 0.112 inch. Any suitable width of the multi-layer adhesivestructure 30 may be used, for example, from about 1 to about 3 feet.Similarly, any suitable length of the multi-layer adhesive structure 30may be used, for example, from about 1 to about 8 feet, or as acontinuous roll.

The multi-layer adhesive structure 30 as shown in FIG. 4 may beconsolidated in an assembly 40 including a vacuum table 41, as shown inFIG. 5. The multi-layer adhesive structure 30 is sandwiched between gaspermeable layers 42 a and 42 b. The gas permeable layers 42 a and 42 bmay comprise sheets of materials such as porous fluorinated ethylenepropylene (FEP). Non-porous sheets 44 a and 44 b are positioned againstthe permeable sheets 42 a and 42 b. The non-permeable sheets 44 a and 44b may be made of any suitable material such as metal, rigid plastic,flexible plastic or the like. Metal plates 46 a and 46 b are positionedoutside the non-permeable sheets 44 a and 44 b. A vacuum bag 48 made ofany suitable material such as nylon or silicone rubber is positionedover the various layers and sealed 49 to the vacuum table 41. Vacuumapplied by the vacuum table 41 creates a vacuum inside the sealed bag48. The adhesive layers 30 a-30 e may be further consolidated byheating, e.g., for 0.5 to 2 hours at a temperature of about 140° F.Without such consolidation at elevated temperature, the adhesive layers30 a-30 e may delaminate during the subsequent forming process.

After the multi-layer adhesive structure 30 has been consolidated, itmay be cut into strips. FIG. 6 illustrates an adhesive strip 32 having aheight H corresponding to the height of the consolidated multi-layeradhesive structure 30 shown in FIG. 5. The adhesive strip 32 has a widthW and a length L. The height H typically is from about 0.05 to about 0.2inch, the width W is typically from about 0.1 to about 0.5 inch, and thelength L is typically from about 1 to about 8 feet or more. For example,H may range from 0.07 to 0.112 inch, W may range from 0.17 to 0.25 inch,and L may range from 30 to 36 inches. The strip 32 may be formed by anysuitable cutting operation. For example, a conventional Gerber cuttermay be used to automatically cut the consolidated multi-layer adhesivestructure 30 into the strips 32. The cutter may utilize a blade orientedat a selected angle, for example, an angle of from about 45 to about 60degrees measured from the plane of the multi-layer adhesive structure30, preferably at an angle of from about 50 to about 55 degrees. Theblade thickness should be sufficient to minimize the possibility ofblade fracture during the adhesive cutting process. A blade thickness of0.04 inch has been found to be suitable.

As shown in FIG. 7, the adhesive strip 32 may be formed into the shapeof a fillet 34 by feeding the strip 32 through opposing rollers 36 and37. Although the rollers 36 and 37 shown in FIG. 7 are opposed to eachother, the rollers may alternatively be offset from each other. One orboth of the rollers 36 and 37 may be driven by a conventional motor (notshown). In accordance with an embodiment of the present invention, acatcher 38 may be positioned adjacent to at least one of the rollers 36and 37 in order to facilitate ejection of the adhesive fillet 34 fromthe rollers 36 and 37. For example, the catcher 38 may be used toprevent the adhesive fillet 34 from sticking to the roller 37 after ithas been formed.

FIG. 8 illustrates a cross section of the adhesive fillet 34. Theadhesive fillet 34 has a base angle A which may range from about 90 toabout 120°, preferably from about 100 to about 110°. For example, anangle A of about 107° may be suitable. An arm of the adhesive fillet 34may have a dimension B of from about 0.1 to about 0.5 inch, preferablyfrom about 0.2 to about 0.3 inch. For example, the dimension B may befrom about 0.22 to about 0.25 inch. The adhesive fillet 34 also has atypical chord distance C of from about 0.2 to about 0.8 inch, preferablyfrom about 0.35 to about 0.5 inch. For example, the chord distance C maybe about 0.36 inch. The adhesive fillet 34 may also be concave, with acurved surface having a radius R of from about 0.1 to about 2 inch ormore, preferably from about 0.2 to about 0.5 inch. For example, theradius R may be about 0.3 inch.

FIG. 9 is a sectional end view illustrating the assembly of a hat-shapedstiffening member 14 on a skin panel 12, including the use of adhesivefillets 16 of the present invention. The adhesive fillets 16 areattached at interior corners 18 of the stiffening member 14. A mandrel50 is positioned on the skin panel 12, and the stiffening member 14 andadhesive fillets 16 are pressed against the mandrel 50 and skin panel12. The stiffening member 14 and adhesive fillets 16, mandrel 50 andskin panel 12 are then placed under a vacuum to ensure intimate contactof the assembly. The assembly is then cured in an autoclave or oven.After the stiffening member 14 and adhesive fillets 16 are secured tothe skin panel 12, the mandrel 50 is removed by sliding it out from thestiffening member 14.

FIGS. 10 and 11 illustrate an opposing roller adhesive fillet formingapparatus 52 in accordance with an embodiment of the present invention.The apparatus 52 includes a lower frame 54, which rotatably supports adrive shaft 56. A motor 58 is connected to the drive shaft 56. The lowerroller 37 is mounted on the drive shaft 56. The motor 58 rotates thedrive shaft 56 and the lower roller 37 at a desired speed, such as 33rpm. An upper frame 64 supports a rotatable shaft 66 upon which theupper roller 36 is mounted. A rotatable handle 68 permits verticalmovement of the upper roller 36 in relation to the lower roller 37. Inthe embodiment shown in FIGS. 10 and 11, the lower roller 37 has arecessed roller surface in the form of a generally V-shaped groove,while the upper roller 36 has an extended roller surface that fits intothe recess of the lower roller 37.

As shown most clearly in FIG. 10, a feed channel 70 extends from thelower and upper frames 54 and 64 inwardly toward the rollers 37 and 36.The catcher 38 is mounted adjacent the rollers 37 and 36 on a supportmember 71, and includes an exit channel 72 through which the adhesivefillet travels after it has been formed by the rollers 37 and 36.

FIG. 12 is an isometric view of a fillet catcher 38 in accordance withan embodiment of the present invention. The exit channel 72 extendsthrough the fillet catcher 38. The fillet catcher 38 includes contouredsurfaces 74 a, 74 b and 76, which substantially conform with the surfaceof the lower roller 37. The fillet catcher 38 also includes contouredsurfaces 78 a and 78 b, which substantially conform with the surface ofthe upper roller 36. As an adhesive fillet is formed by the rollers 37and 36, it is removed from the rollers by the fillet catcher 38. Theformed adhesive fillet contacts the contoured surface 76 of the filletcatcher 38, which extends into the cavity of the lower roller 37. Theformed adhesive fillet then travels through the exit channel 72 of thefillet catcher 38, and exits from the back end 79 of the fillet catcher38.

The following example is intended to illustrate various aspects of thepresent invention, and is not intended to limit the scope of theinvention.

EXAMPLE

Five layers of adhesive sheets were stacked as shown in FIG. 4. Eachsheet had a thickness of 0.014 inch, and was made of FM300 supportedwith scrim cloth. The width of the sheets was 0.23 inch, and the lengthof the sheets was 34 inches. The stacked adhesive sheets wereconsolidated in an assembly similar to that shown in FIG. 5. The sheetswere sandwiched between gas permeable layers made of fluorinatedethylene propylene Teflon film (FEP) sold under the designation Pin HoleA5000. The gas permeable layers were sandwiched between non-permeablesheets made of nonporous Armalon material. Metal plates havingthicknesses of 0.125 inch were positioned as shown in FIG. 5, and theassembly was covered with a sheet of porous polyester sold under thedesignation Air Weave and a nylon vacuum bag. The bag was sealed to avacuum table by a seal made of nylon bagging material sold under thedesignation Airtech. The assembly was heated to a temperature of 140° F.under a vacuum of 26 inches of mercury and held for 1 hour. The pressureapplied by the plates was about 12 psi. At temperatures above about 140°F., the adhesive layers may not sufficiently consolidate, while attemperatures above 140° F. the adhesive may begin to cure, makingsubsequent forming difficult. The consolidated adhesive layers were thencooled to room temperature and stored under vacuum.

Upon removal from the assembly, the consolidated multi-layer adhesivestructure had a thickness of about 0.075 inch. It was then cut in aGerber cutter with a cutting blade angle of 55° to form multi-layeradhesive strips having heights of 0.075 inch, widths of 0.23 inch andlengths of 38 inches.

Each strip was then formed into an adhesive fillet shape by passing thestrips through a roll-former as illustrated in FIGS. 10 and 11. Theroll-former was equipped with a catcher as illustrated in FIG. 12, whichprevented the formed adhesive fillet from sticking to the lower roller.

The formed adhesive fillet had a cross section similar to that shown inFIG. 8, having an angle A of 107.14°, an arm dimension B of 0.226 inch,a chord length C of 0.363 inch, and a radius R of 0.306 inch. During theroll-forming operation, the cross sectional area of the adhesive filletwas reduced approximately 60 percent in comparison with the initialcross sectional area of the multi-layer adhesive strip, resulting incompaction, deformation and elongation of the original strip by about 20percent. If the cross sectional area of the adhesive fillet is reducedby 200 percent or more, it has been found that the adhesive filletelongates over 30 percent and forms an irregular shaped fillet. Afterformation, the adhesive fillet may be sandwiched between a mandrel hatand skip panel, e.g., as shown in FIG. 9.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1. A formed adhesive fillet comprising multiple stacked and consolidatedadhesive layers, wherein the consolidated adhesive layers consistessentially of epoxy adhesive and do not include fibers.
 2. The formedadhesive fillet of claim 1, wherein the fillet comprises from 5 to 8 ofthe stacked adhesive layers.
 3. The formed adhesive fillet of claim 1,wherein the fillet has a cross section including two arms oriented at anangle of from about 90 to about 120 degrees with respect to each other.4. The formed adhesive fillet of claim 2, wherein each of the arms has alength of from about 0.1 to about 0.5 inch.
 5. The formed adhesivefillet of claim 1, wherein the fillet comprises a concave portion havinga radius of curvature of from about 0.1 to about 2 inches.
 6. The formedadhesive fillet of claim 5, wherein the fillet comprises two arms andhas a chord length measured between tips of the arms of from about 0.2to about 0.8 inch.
 7. A stiffened panel comprising: a panel; astiffening member attached to the panel; and at least one formedadhesive fillet attached to the panel and the stiffening member, whereinthe at least one formed adhesive fillet comprises multiple stacked andconsolidated adhesive layers, wherein the consolidated adhesive layersconsist essentially of epoxy adhesive and do not include fibers.
 8. Thestiffened panel of claim 7, wherein the fillet comprises a 5 to 8 of thestacked adhesive layers.
 9. The stiffened panel of claim 7, wherein thepanel comprises an aircraft panel having a thickness of from about 0.1to about 0.2 inch.
 10. The stiffened panel of claim 9, wherein the panelcomprises a graphite/epoxy composite.
 11. The stiffened panel of claim7, wherein the stiffening member comprises a substantially hat-shapedcross-section.
 12. The stiffened panel of claim 11, wherein thestiffening member has a thickness of from about 0.1 to about 0.2 inch, awidth of from about 2 to about 5 inches, and a height of from about 0.5to about 1.5 inches.
 13. The stiffened panel of claim 11, wherein theadhesive fillets are attached at interior corners where the stiffenermeets the panel.